The assembly of two electronic components by the so-called “flip-chip” technique, for example, by thermocompression, usually comprises forming electrically-conductive solder balls on a surface of a first electronic component and on a surface of a second component according to a predetermined connection pattern. The first component is then arranged on the second component to align their respective solder balls, after which the assembly is pressed and heated. The balls placed into contact then deform and melt to form electrical interconnects perpendicular to the main plane of the electronic components, generally in the form of a slice.
A device comprising two electronic components separated by a distance in the range from 1 micrometer to 10 micrometers, having a mutually facing area greater than 100 mm2 (for example, two square surfaces with a 10-millimeter side length facing each other), is thus generally obtained. Usually, the surface density of interconnects is in the range from 1010/m2 and 1012/m2.
A problem with this type of assembly is that the vertical interconnects obtained by the hybridization are sensitive to thermal stress, and this, all the more as the first and second components are made of different materials. Indeed, the components most often have different thermal expansion coefficients, so that under the effect of a temperature variations, the interconnects are submitted to a shearing which embrittles them and breaks them.
To increase the thermo-mechanical reliability of a hybridized assembly and to provide a protection of interconnects against the environment, it is generally provided to fill the space separating the two components with a resin layer known as “underfill”, the action of filling this space being known as “underfilling”. The shearing forces are thus distributed all over the layer separating the two hybridized components, and no longer on interconnects only, the latter being thus efficiently protected. It is then spoken of an “encapsulated flip-chip”. Reference may for example be made to document “Underfill material selection for flip chip technology” of Diana C. Chiang, Thesis (S.M.), Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998.
Two techniques for filling the volume separating the two components hybridized with solder balls are known: the first one is known as “fast flow”, and the second one is known as “no-flow”. Such techniques are for example described in document “Characterization of a No-Flow Underfill Encapsulant During the Solder Reflow Process”, of C. P. Wong et al., Proceedings of the Electronic Components and Technology Conference, 1998, pages 1253-1259.
A “flip-chip” assembly followed by an underfill according to the “fast flow” technique is now described in relation with FIGS. 1 to 4.
In a first step (FIG. 1), a first electronic component 10a provided with solder balls 12a on one of its surfaces 14a, is aligned with a second electronic component 10b, also provided with solder balls 12b on one of its surfaces 14b. A pressure is then exerted on the second component as indicated by the illustrated arrows by further raising the temperature of the assembly to a temperature higher than or equal to the melting temperature of the metal forming balls 12a, 12b. Balls 12a, 12b then bond to one another by thermocompression to form interconnects 16 (FIG. 2). During a next step (FIG. 3) usually following the application of a deoxidation flow to clean interconnects 16, an electrically-insulating liquid resin 18 is deposited by means of a dispenser 20 on surface 14a of first component 10a. Resin 18 then migrates by capillarity into volume 20 separating the opposite areas of parallel surfaces 14a, 14b and ends up totally filling this volume 20, thus embedding electrical interconnects 16 (FIG. 4). Resin 18 is then solidified, usually by application of a thermal treatment, or “curing”. A last step of connecting the hybridized device to outer elements (not shown) is then implemented (FIG. 5), for example, by connecting connection areas 22, provided for this purpose on first component 10a, with wires 24 (so-called “wire bonding” connections).
As known per se, the resin is a mixture of glue as a main component, for example, epoxy glue, and of a solvent which enables to adjust the viscosity of the resin and which is evaporated during the thermal treatment of the resin. The mixture may also comprise hardening agents, particularly polymerizing agents, for example, a catalyst, a photoinitiator or a thermal initiator, and/or surface agents, for example, silane, which increases the bonding and the wettability of the resin on the surfaces of the components with which it enters into contact, and/or particles for adjusting the thermal expansion coefficient of the resin, usually called “fillers”.
The first problem posed by the “encapsulated flip chip” technique is that of the presence of a polymer in the filling resin. Now, polymers are by nature “non tight”, that is, they cannot form on the long term a barrier against humidity. Further, their efficiency against humidity strongly decreases when the device is submitted to significant thermal excursions. More particularly, a corrosion of interconnects 16 in the presence of the adsorbed humidity can be observed. Indeed, interconnects are generally formed of a complex stack of metallic materials (solders, intermetallic, bonding metals, solder diffusion barrier metals, etc.) whereby such structures have chemical potentials favoring an accelerated corrosion in the presence of humidity.
Further, humidity implies the swelling of the encapsulation resins after humidity has been absorbed, which induces mechanical stress tending to separate the components and resulting in prematurely breaking interconnects 16.
Thus, the encapsulation material of the state of the art alone does not provide a good resistance to climatic stress.